Deposition chamber and method for low dielectric constant film

0001The invention is directed to a film deposition method and a deposition system for carrying out the method.

0002One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on a semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition (CVD). Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions can take place to produce the desired film. High density plasma CVD processes promote the disassociation of the reactant gases by the application of radio frequency (RF) energy to the reaction zone proximate the substrate surface thereby creating a plasma of highly reactive species. The high reactivity of the released species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes.

0003In one design of high density plasma chemical vapor deposition (HDP-CVD) chambers, the vacuum chamber is generally defined by a planar substrate support, acting as a cathode, along the bottom, a planar anode along the top, a relatively short sidewall extending upwardly from the bottom, and a dielectric dome connecting the sidewall with the top. Inductive coils are mounted about the dome and are connected to a supply radio frequency generator. The anode and the cathode are typically coupled to bias radio frequency generators. Two or more sets of equally spaced gas distributors, such as nozzles, are typically mounted to the sidewall and extend into the region above the edge of the substrate support surface. The gas nozzles for each set are coupled to a common manifold for that set; the manifolds provide the gas nozzles with process gases, including gases such as argon, oxygen, silane (SiH₄), TEOS (tetraethoxy-silane), silicon tetrafluoride (SiF₄), etc., the composition of the gases depending primarily on the type of material to be formed on the substrate. Sets of gas nozzles are commonly used because some gases, such as silane, need to be delivered into the chamber separately from other gases, such as oxygen; other gases, such as oxygen and SiF₄, can be delivered to a common set of nozzles through a common manifold. The nozzle tips have exits, typically orifices, positioned in a circumferential pattern spaced apart above the circumferential periphery of the substrate support and through which the process gases flow.

0004As semiconductor devices shrink in size, the capacitance between adjacent conductors increases. This increase in capacitance adversely affects the speed of the devices. To partially alleviate this problem, the dielectric constant of the deposited dielectric film, typically about 4.1 for undoped silicon glass (USG), must be reduced. Recently, fluorine doping with SiF4 chemistry, which results in fluorine-doped silicon glass (FSG), has become popular to reduce the dielectric constant. It is believed that it is possible to produce a dielectric film having good thermal stability with a dielectric constant of 3.5 with FSG. However, this value (3.5) for the dielectric constant has not been readily achievable.

0005EP-A-0 599 730 discloses a method for forming silicone oxide films having a dielectric constants of 3.7 to 2.9. This document shows a remote plasma system. Argon and oxygen are introduced into a plasma chamber, and from the plasma chamber the argon/O2-plasma is introduced to the deposition chamber, to which silicone tetraflouride is supplied.

0006Furthermore, US-A-5,304,250 discloses a deposition chamber having a top with a first gas injection manifold having one single connector and a concentric plenum having exits at a plurality of positions spaces apart and equally spaced about the center of the top. A second gas manifold is provided comprising a plurality of nozzles configured to injecting gaseous substance into the chamber. The nozzles are surrounding the substrate at a plurality of position and are located generally above the substrate.

0007From the WO-A-97/032224 and EP-A-0839217 which is state for art according to Art. 54 (3) EPC with a publication date of January 30, 1997, a filing date of June 21, 1996 and a priority date of July 10, 1995, a plasma enhanced chemical processing reactor and method is known, the reactor including a plasma chamber having a top with a first gas injection manifold, a second gas manifold and a source of electromagnetic energy. The first gas injection manifold having independent connectors and concentric and circumferentially extending plenums each having exits at a plurality of positions spaces apart and equally spaced about the center of the top. The second gas manifold is mounted along the wall of the processing chamber and comprises a nozzle structure which has a plurality of nozzles configured to injecting gaseous substances into the chamber. The gas distributors and exits are surrounding the substrate at a plurality of position and are located generally above the substrate.

0008It is the object of the present invention to provide a method and a deposition system for depositing uniform films with uniformly low dielectric constants of below 3.5, preferably below 3.4, and more preferably below 3.3.

0009The above object is achieved by a method for depositing a film onto a substrate within a deposition chamber according to claim 1 and by a deposition system according to claim 11. The dependent claims are related to further advantageous aspects of the present invention.

0010This is achieved by a the uniform application of the gases (preferably silage, fluorine-supplying gases such as SiF4 or CF4, and oxygen-supplying gases such as O2 or N2O) to the substrate. The result is preferably improved by the selection of optimal flow rates for the gases, which preferably have been determined as a result of tests using the particular chamber.

0011The improved deposition system according to a preferred embodiment of the invention includes a housing defining a deposition chamber. A substrate support is housed within the deposition chamber. A first gas distributor has orifices or other exits opening into the deposition chamber in a circumferential pattern spaced apart from and generally overlying the circumferential periphery of the substrate support surface. In one embodiment the first gas distributor comprises first and second sets of nozzles. Oxygen and SiF4 are preferably delivered together to the chamber through the first set of nozzles and silane (or silane and SiF4) is delivered through the second set of nozzles. Mixing the SiF4 with oxygen and introducing this combination through the first set of nozzles reduces equipment complexity so cost can be reduced. A second gas distributor, preferably a center nozzle, is used and is positioned spaced apart from and above the substrate support surface. The use of the second gas distributor to inject a gas, preferably silane (or silane and SiF₄), into the vacuum chamber helps to improve the uniform application of the gases to the substrate over that which is achieved without the use of the second gas distributor.

0012Oxygen-supplying gas (preferably oxygen) is delivered to the chamber through the top of the housing in a region generally above and preferably generally centrally above the substrate. This is preferably achieved by passing the oxygen through an annular orifice created between the center nozzle carrying the silane (and any other gases) and a hole in the top of the housing. In this way, oxygen is provided both from the sides through the first set of nozzles of the first gas distributors, preferably mixed with SiF₄, and also in the same region as silane above the substrate. Also, the passage of the oxygen through the annular orifice keeps reactive gases within the chamber from attacking the seals used between the top of the housing and the body from which the center nozzle extends. This advantage is retained if silane is passed through the annular orifice and oxygen through the center nozzle.

0013Film thickness and dielectric constant uniformity is also enhanced by insuring that the temperature of the substrate remains uniform across the substrate and using a source RF generator designed to achieve sputtering uniformity.

0014One of the primary aspects of the invention is the recognition that it is very important to ensure the uniform distribution of oxygen entering the chamber. This is achieved by flowing oxygen both from the top of the chamber and from the sides of the chamber. Additionally, by the appropriate configuration of the oxygen flow path through the top of the chamber, the oxygen can serve to protect the sealing element from deleterious effects of coming in contact with reactive gases such as fluorine.

0015In addition to the need to supply the gases to the substrate uniformly, it is preferable to use the appropriate proportion of the gases, typically oxygen, silane and SiF₄, to achieve the minimum dielectric constant. The proper flow rates for each will differ according to the particular chamber used. Accordingly, it is a preferred aspect of the invention to test a variety of flow rate proportions to determine which set of flow rates provides a high quality dielectric film with a minimum dielectric constant.

0016Further preferred embodiments of the invention and the features thereof are given in the appended claims and subclaims.

0017Preferred embodiments of the invention will now be described in detail in conjunction with the accompanying drawings in which <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a schematic cross-sectional view showing a deposition chamber made according to one embodiment of the invention;</li><li>Fig. 2 is an exaggerated view illustrating the characteristic M-shaped, deposition thickness variation plot of the prior art;</li><li>Fig. 3 illustrates an improvement in the deposition thickness variation plot of Fig. 2 using the apparatus of Fig. 1;</li><li>Fig. 4 is a graph of dielectric constant versus oxygen flow for different flow rate ratios of SiF₄ to silane;</li><li>Fig. 5 is a simplified view of an alternative embodiment of the center nozzle of Fig. 1 having three orifices; and</li><li>Fig. 6 is a view in the region of the center nozzle showing additional oxygen passageways.</li></ul>

0018Fig. 1 illustrates a deposition chamber 2 comprising a housing 4, the housing including a generally cylindrized dielectric enclosure 6 surrounded by two sets of RF inductive coils 8, 9. Enclosure 6 could be made of RF transparent materials other than a dielectric material. Coils 8, 9 are powered by a pair of source RF generators 10, 11. Chamber 2 also includes a water-cooled substrate support 14 having a substrate support surface 16 within the vacuum chamber 18 defined within housing 4. Surface 16 is used to support a substrate 20 within chamber 18. Substrate support 14 acts as a cathode and is connected to a bias RF generator 22 through a matching circuit 24. A generally cylindrical sidewall 30 of housing 4 connects the bottom 32 of housing 4 to dielectric enclosure 6. Sidewall 30 acts as the anode.

0019Process gases are introduced to vacuum chamber 18 in the region surrounding substrate 20 through two sets of 12 equally spaced nozzles 34, 34a. Nozzles 34, 34a are arranged in a ring-like pattern and are fluidly coupled to gas manifolds 36, 36a, respectively. Manifolds 36, 36a are fed process gases from first and second gas sources 35, 35a through first and second gas controllers 37, 37a and first and second gas feed lines 39, 39a. Each nozzle 34, 34a has an orifice 38 at its distal end. The orifices 38 of nozzles 34, 34a are arranged above the periphery 40 of substrate support 14 and thus above the periphery 42 of substrate 20. Vacuum chamber 18 is exhausted through an exhaust port 44.

0020Fig. 2 illustrates a typical USG deposition thickness variation plot 46 for a conventional deposition chamber. The average thickness is shown by base line 48. As can be seen by plot 46, there is a relatively steep increase in thickness at end points 50 and 52 of plot 46 corresponding to the periphery 42 of substrate 20. The center 54 of plot 46 also dips down substantially as well.

0021Plot 46 can be improved through the use of a center nozzle 56 coupled to a third gas source 58 through a third gas controller 60 and a third gas feed line 62. Center nozzle 56 has an orifice 64 positioned centrally above substrate support surface 16. Using center nozzle 56 permits the modification of USG deposition thickness variation plot 46 from that of Fig. 2 to exemplary plot 68 of Fig. 3. Exemplary deposition thickness variation plot 68 is flat enough so that the standard deviation of the deposition thickness can be about 1 to 2% of one sigma. This is achieved primarily by reducing the steep slope of the plot at end points 50, 52 and raising in the low point at center 54 of plot 46.

0022The various components of chamber 2 are controlled by a processor (not shown). The processor operates under control of a computer program stored in a computer-readable medium (also not shown). The computer program dictates the various operating parameters, such as timing, mixture of gases, chamber pressure, substrate support temperature and RF power levels.

0023The embodiments of the present invention improve upon the above-described structure in a manner which results in films having dielectric constants of less than 3.5 and preferably less than 3.4 and more preferably less than 3.3; these low dielectric constant values are achieved in a generally uniform manner over substrate 20. The uniform reduction of the dielectric constant is important because as device sizes are reduced, capacitance between closely spaced conductors will naturally increase. To reduce the capacitance, and thus speed up operation of the devices, the dielectric constant of the deposited dielectric film must be reduced.

0024In the embodiments of the present invention preferably a combination of SiF₄ and oxygen from first gas source 35 for introduction into chamber 18 is supplied through orifices 38 of nozzles 34. Doing so simplifies the delivery of these gases and helps reduce cost. Silane (SiH₄) is preferably delivered into chamber 18 from second gas source 35a, through second gas controller 37a, and through nozzles 34a. In addition, third gas source 58 is preferably used to introduce silane (or, for example, a mixture of silane and SiF₄) into chamber 18 from above substrate 20. In conjunction with this, oxygen is also directed into chamber 18 from a position above substrate 20 but along a flow path separate from the flow path of the silane.

0025Oxygen can be mixed with a relatively stable gas such as SiF₄; however, due to the reactive nature of silane and oxygen, these components must be kept separate until their introduction into chamber 18. To accomplish this, separate nozzles 34, 34a are used in the region around substrate support 14; also an oxygen pathway 70 is formed in a body 72 mounted to the top 75 of enclosure 6. Pathway 70 is coupled to an oxygen source 71 through an oxygen controller 73. Third gas line 62 passes through body 72 and terminates at center nozzle 56. Center nozzle 56 passes through an opening 74 formed in top 75. Nozzle 56 and opening 74 provide an annular orifice 76 in fluid communication with vacuum chamber 18 and oxygen pathway 70. A fluid seal 78 is provided between body 72 and top 75. Oxygen thus proceeds through pathway 70, into a region defined between body 72 and top 75 and bounded by fluid seal 78, and finally along annular orifice 76. By injecting oxygen in this way, gases, such as fluorine compounds, which could otherwise have a deleterious effect on fluid seal 78, are prevented from reaching the fluid seal by the washing effect or scouring effect of the flowing oxygen. Gases other than oxygen which do not cause seal 78 to deteriorate can also be used.

0026In conjunction with the uniformity of gas distribution using the structure discussed above, uniform dielectric constants are also dependent upon temperature uniformity across substrate 20 and sputtering uniformity. Varying the total flow of SiF₄ and silane affects deposition rate and thus throughput. High throughput requires high bias power from bias power source 22 to create high sputtering and high etching rates. High bias power, and thus high throughput, is possible only if temperature uniformity across substrate 20 is achieved since speed of etching is strongly affected by the temperature of the substrate.

0027The determination of the amounts of SiF₄, silane (SiH₄) and oxygen to be used creates an entire new layer of complexity. Assuming the total flow rate of silicon (e.g., from SiH₄ and SiF₄) remains constant, it is believed that several basic statements can be made regarding the use of these various components. If too little oxygen is used, the deposition rate drops dramatically thus making the process much too inefficient. Too little oxygen can leave the film silicon rich with excess free fluorine incorporated into the film. If too much oxygen is used, the resulting film becomes more USG and the dielectric constant becomes high. If too much SiF₄ is used, aging problems can result; aging problems result because over time the fluorine, which is not bound tightly in the complex chemistry of the resulting film, gets released causing deterioration of the device. Too much silane will cause the film to behave more like USG and thus result in a dielectric constant at an undesirable level.

0028The optimal amounts of oxygen, SiF₄ and silane at the substrate surface are the stoichiometric proportions. However, flowing stoichiometric proportions of the gases into deposition chambers, including chamber 2 and other deposition chambers, would result in gas proportions at the substrate surface which are not the stoichiometric proportions. The actual proportions of the gas flowing into the deposition chamber needed to achieve stoichiometric proportions at the substrate surface will vary from the stoichiometric proportions at least in part according to the structure of the specific chamber. The more efficient the chamber, the less gas is wasted so that gas flow rates closer to the stoichiometric amounts can be used.

0029To determine the proper relative flow rates of SiF₄, silane and oxygen for a particular chamber to achieve the desirable dielectric constant below 3.5, preferably below 3.4 and more preferably below 3.3, the proportions of the three components could be varied in any desired manner to create a number of dielectric films on substrates 20; the dielectric constant at different positions along each dielectric film could then be measured. However, some limits in the relative amounts are in order. The percentage of SiF₄ should be between about 40% to 60% of the total silicon-supplying gas to reduce or eliminate the problems resulting from too much or too little SiF₄ and silane. Oxygen should be between about 60% to 100% of the total silicon-supplying gas.

0030Fig. 4 illustrates the results of a set of tests conducted varying the ratios of SiF₄ to silane to oxygen. It was found that by selecting a total reactive gas flow rate, that is a flow rate for the combination of SiF₄ and silane (which results in a constant amount of silicon), dividing that total between SiF₄ and silane to arrive at various proportions of SiF₄ and silane, and then, using those proportions, varying the oxygen flow, the graph shown in Fig. 4 of dielectric constant to oxygen flow was created. This type of graph provides very useful data.

0031Plot A, resulting from 44 sccm SiF₄ to 36.4 sccm silane, results in a dielectric constant which varies from 3.4 at an oxygen flow of about 62 sccm to about 3.8 at an oxygen flow rate of about 110 sccm. It is not clear from this graph where the minimum dielectric constant would be for this ratio of SiF₄ to silane. It appears, however, that the minimum would occur at an unacceptably low oxygen flow rate. Plot B, having an sccm flow rate ratio of SiF₄ to silane of 36 to 44.4 provides the lowest dielectric constant: about 3.2 at an oxygen flow of 60 sccm. Plots C and D have minimum dielectric constants of about 3.5 and 3.6 respectively. From this graph it is clear that for these particular ratios of SiF₄ to silane, the ratio for plot B provides the lowest dielectric constant with oxygen flow being at an acceptable level. Reviewing plots A and B suggests that a proportion of SiF₄ to silane between the proportions for these two plots may yield a lower dielectric constant than achievable with the proportion for plot B.

0032Accordingly, the embodiments of the present invention provide a useful and efficient way of determining how to achieve films with low dielectric constants using SiF₄ (or another fluorine-supplying gas) and silane chemistry to achieve the reduced dielectric constants. While the above-described method of choosing a single total reactive gas flow rate for each of the tests is presently preferred, other methods for the orderly gathering of dielectric constant information may also be pursued. For example, it may be desired to allow all three variables to change within the overall parameters.

0033In use, a film having a low dielectric constant can be deposited on substrate 20 by first determining the appropriate flow rates of SiF₄, silane and oxygen, typically in the manner discussed above by plotting the results of different tests. Once the desired rate for the particular chamber has been determined, silane is introduced into chamber 18 from second gas source 35a, a mixture of silane and SiF₄ is introduced into chamber 18 from third gas source 58, oxygen is introduced into the chamber from oxygen source 71, and a mixture of oxygen and SiF₄ is introduced into chamber 18 from first gas source 35. Argon is also introduced from first and third sources 35, 58. Deposition uniformity is also aided by insuring that the temperature of substrate 20 is uniformly controlled over its surface and by the use of a variable frequency source RF generators 10, 11 to help achieve uniform sputtering.

0034The above-described embodiment has been designed for substrates 20 having diameters of 8 inches (20 cm). Larger diameter substrates, such as substrates having diameters of 12 inches (30 cm), may call for the use of multiple center nozzles 56a as illustrated in Fig. 5 by the nozzle assembly 56'. In such embodiments the deposition thickness variation plot would likely have a three-bump (as in Fig. 3), a four-bump or a five-bump shape. The particular shape for the deposition thickness plot would be influenced by the type, number, orientation and spacing of center nozzles 56A and orifices 64.

0035In addition to orifice 76, oxygen may also be directed into chamber 18 through a number of downwardly and outwardly extending passageways 80 as shown in Fig. 6. Each passageway 80 has an orifice 82 where oxygen enters into chamber 18. If desired, other gases, such as argon, may be mixed with one or both of the silane passing through orifice 64 or oxygen passing through annular orifice 76 or orifices 82.